Image sensors are provided with sensing elements which provide a potential well or depletion region in a substrate. Color filter arrays having patterns of color are selectively formed over the sensing elements. Light passes through these colored patterns and causes charge to be collected in the potential wells.
Color filter arrays are presently fabricated on top of photoelectronic sensors by patterning a binder resin containing a mordant having a charge. The pattern is then subjected to a solution of a dye having the opposite charge. This results in dye incorporation into the pattern due to the binding of the oppositely charged species. The full color filter array requires multiple colors. Color filter arrays are generally classified into two types, i.e., primary color type filter array employing red, green, and blue (RGB), and complementary color type filter array employing white, yellow, and cyan (WYC), or white, yellow, cyan, and green (WYCG), or the like.
The image sensor is able to reproduce a color image by combining the signal levels from all the individual color sensing elements (pixels). Both the signal level and the known spectral sensitivity of the element are needed to accurately reproduce a color. It is advantageous to have the sensitivity of each element the same. Thus the red element is as sensitive to red light as the green element is sensitive to green light and so forth. The sensitivities can be matched electronically during the processing however this requires gain adjustments to each of the color channels. The gain adjustments introduce noise into the picture since both signal and noise are effected by gain adjustments. Therefore, it is advantageous to have the transmittance of the various filter elements be the same in their respective regions of the visible spectrum.
The level of transmittance is determined by both the amount of dye in the binder and the intrinsic absorptivity of the dye. The intrinsic absorptivity of a dye can not be changed therefore varying the amount of dye in the patterned binder is the only way of altering the transmittance. The amount of dye imbibed will be a function of the time the dye solution is left in contact with the binder and the total number of mordant sites in the binder. Stopping the dyeing process at some point short of saturation introduces a source of variability making it hard to accurately control the final dye density. Adjusting the number of mordant sites in the binder was used by Reithel and Sutton (U.S. Pat. No. 4,942,103). This method requires balancing the ratio of patterning component to mordant to obtain a compromise between photospeed, thickness, and dye density. It is likely using this method that a separate binder formulation will be needed for each dyed layer in a multilayer color filter. The preferred procedure is to saturate the binder with dye and control the transmittance by varying the thickness of the binder layer as is taught by Blood and Pace (U.S. Pat. No. 4,764,670). This can be done by controlling the spin speed used to coat the binder. This method is widely used and works quite well in most cases. However, there are cases where the desired thickness lies outside the range obtainable with spin coating equipment. If thicker coatings are needed, multiple layers could be coated however if thinner coatings are needed there are no reliable methods presently available. The lower thickness limit could also be dictated by the amount of surface topography present on the substrate containing the image sensor. Surface roughness will cause thickness non-uniformities in coatings applied by spinning. As the thickness decreases, the percent variability seen due to surface features interfering with the flow across the substrate surface increases. Thickness variations lead to dye density variations which will cause unacceptable color reproduction.